MIC2168A_15 Datasheet, PDF (9/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck-Control IC

Micrel, Inc.
MIC2168A
Figure 1. MIC2168A Current Limiting Circuit
The current limiting resistor R_CS is calculated by the
following equation:
R _CS
=
R_DS(ON)Q1 × I_L
200µA
I_L
= I_LOAD
+
1
2
× I_PP
where:
I_PP = Inductor Ripple Current =
V_{OUT ×} V_IN − V_OUT
V_IN F_SWITCHING × L
F_SWITCHING = 1MHz
200μA is the internal sink current to program the
MIC2168A current limit.
The MOSFET R_DS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add a 50%
margin to the load current (I_LOAD) in the above equation
to avoid false current limiting due to increased MOSFET
junction temperature rise. It is also recommended to
connect R_CS resistor directly to the drain of the top
MOSFET Q1, and the R_SW resistor to the source of Q1
to accurately sense the MOSFETs R_DS(ON). To make the
MIC2168A insensitive to board layout and noise, a 1.4Ω
resistor and a 1000pF capacitor is recommended below
the switch node and ground. A 0.1μF capacitor in
parallel with R_CS should be connected to filter some of
the switching noise.
Internal V_DD Supply
The MIC2168A controller internally generates V_DD for
self biasing and to provide power to the gate drives. This
V_DD supply is generated through a low-dropout regulator
and generates 5V from V_IN supply greater than 5V. For
supply voltage less than 5V, the V_DD linear regulator is
approximately 200mV in dropout. Therefore, it is
recommended to short the V_DD supply to the input
supply through a 10Ω resistor for input supplies between
2.9V to 5V.
MOSFET Gate Drive
The MIC2168A high-side drive circuit is designed to
switch an N-Channel MOSFET. The Functional Diagram
shows a bootstrap circuit, consisting of D1 and CBST,
supplies energy to the high-side drive circuit. Capacitor
CBST is charged while the low-side MOSFET is on and
the voltage on the VSW pin is approximately 0V. When
the high-side MOSFET driver is turned on, energy from
CBST is used to turn the MOSFET on. As the MOSFET
turns on, the voltage on the VSW pin increases to
approximately V_IN. Diode D1 is reversed biased and
CBST floats high while continuing to keep the high-side
MOSFET on. When the low-side switch is turned back
on, CBST is recharged through D1. The drive voltage is
derived from the internal 5V V_DD bias supply. The
nominal low-side gate drive voltage is 5V and the
nominal high-side gate drive voltage is approximately
4.5V due the voltage drop across D1. An approximate
20ns delay between the high- and low-side driver
transitions is used to prevent current from
simultaneously flowing unimpeded through both
MOSFETs.
MOSFET Selection
The MIC2168A controller works from input voltages of
3V to 13.2V and has an internal 5V regulator to provide
power to turn the external N-Channel power MOSFETs
for high- and low-side switches. For applications where
V_IN < 5V, the internal V_DD regulator operates in dropout
mode, and it is necessary that the power MOSFETs
used are low threshold and are in full conduction mode
for V_GS of 2.5V. For applications when V_IN > 5V; logic-
level MOSFETs, whose operation is specified at V_GS =
4.5V must be used.
It is important to note the on-resistance of a MOSFET
increases with increasing temperature. A 75°C rise in
junction temperature will increase the channel
resistance of the MOSFET by 50% to 75% of the
resistance specified at 25°C. This change in resistance
must be accounted for when calculating MOSFET power
dissipation and in calculating the value of current-sense
(CS) resistor.
April 22, 2015
9
Revision 3.0